1. Field of the Invention
The invention relates to Ziegler-Natta catalysts useful for olefin polymerizations. More particularly, it relates to multi-metallic Ziegler-Natta catalysts showing improved efficiency and properties in production of ethylene based polymers such as linear low density polyethylene (LLDPE) polymers in particular.
2. Background of the Art
Currently the demand for polyethylene worldwide is in excess of 80 million metric tons per year. Because there is a need for significant and continued differentiation of polymer products in the polyethylene business, researchers have devoted a great deal of effort to searching for process alterations that will yield such new products. One focus involves exploring new catalysts.
Ziegler-Natta catalysts have been used for many years in producing a variety of polyethylenes, including LLDPE. These catalysts generally include a magnesium halide support and one or two transition metal compounds. Though effective, these catalysts frequently result in LLDPE resins with broad polydispersity and undesirably broad short chain branching distribution (SCBD).
In LLDPE production, ethylene is generally copolymerized with a short-chain olefin comonomer (for example, 1-butene, 1-hexene and/or 1-octene). The resulting polymer is substantially linear but includes significant numbers of short branches, and these characteristics give it a higher tensile strength, higher impact strength and higher puncture resistance than those of low density polyethylene (LDPE). These improved properties, in turn, mean that lower thickness (gauge) films can be blown and the product exhibits improved environmental stress cracking resistance. LLDPE is used predominantly in film applications due to its toughness, flexibility and relative transparency. Product examples range from agricultural films, food protection wrap, and bubble wrap, to multilayer and composite films. Unfortunately, LLDPE tends to be somewhat more difficult overall to process than LDPE and may also be less tough and/or exhibit some less desirable optical properties.
In view of the above deficits in the performance of many known LLDPE polymers, it is desirable to identify process and/or compositional means and methods which can be employed in producing new, differentiated LLDPE polymers. Furthermore, it is desirable that such new means and/or methods offer process and product advancements such as enhanced catalyst efficiencies, narrower polydispersities, and narrower SCBD for improved uniformity of such products.